Amplitude limiting circuits



Jan. 4, 1944. G. l. DALLOS 2,338,412

AMPLITUDE LIMITING CIRCUITS Filed March 29, 1940 5 Shee tsSheet l A TaRA/Ey Jan. 4, 1944. e. 1. DALLOS AMPLITUDE LIMITING CIRCUITS 5 Sheets-Sheet 2 Filed March 29, 1940 Jan. 4, 1944. G. DALLOS AMPLITUDE LIMITING CIRCUITS 5 Sheets-Sheet 3 Filed March 29, 1940 9mm; @an

f S Wm W ed d/ v Jan. 4, 1944. 3 L DALLQS 2,338,412

AMPLITUDE LIMITING CIRCUITS Filed March 29, 1940 5 Sheets-Sheet 4 ywowm Jan. 4, 1944. G. 1. DALLOS 2,338,412

AMPLITUDE LIMITING CIRCUITS Filed March 29, 1940 I 5 Sheets-Sheet 5 FVGIG am m 6 any; gofmin @4366 moRM y Patented Jan. 4, 1944 OFFICE 2,338,412 AMPLITUDE N LIMITING cmourrs Gyiirgy Istvan Dallas, Budapest, Hungary; vested in the Alien Property Custodian Application March 29, 1940, Serial No.

In Hungary March 23, 1939 13 Claims.

There are many known circuits for limiting the amplitude 01' an alternating current to a certain limiting value for the purpose of excluding interference peaks, for instance in radio reception. For this limiting actionelectron tubes having a grid have been proposed. These tubes have the drawback that even for signal amplitudes suited for transmission without exceeding the range of control of the tubes the curvature of the bend of the tube characteristic seriously impairs the sharpness of the cut oil, so that the harmful interference peaks are also transmitted, at least to a certain extent. It is also impossible to obtain a sharp limitation by means of those known methods in which a diode bridged across an impedance, for instance an electron tube, shortcircuits said impedance on the occurrence of an interference. As the resistance of a diode is of the order of magnitude of 1000 ohms the short circuit will not be complete. Further the sudden change of impedance may cause serious trouble. For the purpose of eliminating interference it is also known to employ a diode which is supplied with current from a rectifier and prevents transmission of the greatest signal amplitudes, for instance those exceeding 100% modulation. With circuits of this kind it is merely possible to obtain a slight reduction of interference and the distortion is usually great.

Further it has been proposed to employ amplitude limiting circuits in which the alternating current to be limited is passed through two electron tubes which are provided with control grids and are connected in tandem from an A. C. point of View. The tubes are connected in such a manner that as soon as the input voltage reaches a predetermined limiting value grid current is produced preventing the anode current from further increasing. One of the tandem connected tubes is for limiting one half wave of the alternating current and the other is for limiting the other half wave. The drawback of this circuit is that the operation is not re liable and that the limiting operation is at tended by distortion.

The invention relates to an amplitude limiting circuit of the kind in which the alternating current or voltage to be limited is passed through two tubes connected in tandem from an A. C. point of view. The invention is characterised in that the tubes referred to are diodes the D. C. and A. C. circuits of which are controlled independently, further in that the first diode is so connected to the second diode that the alternating voltage to be limited is supplied to the two of higher degree.

diodes in opposite directions, and in that the diodes are supplied with a bias voltage (prefery proportional to the amplitude of the voltage to be limited) of such magnitude that, whereas the alternating voltage to be limited will just be passed by the diodes without rectification, the first diode will cut oil the amplitude peaks exceeding the limiting value and being superimposed on one of the half waves of the voltage v to be limited, and the second diode will cut oil the amplitude peaks exceeding the limiting value and being superimposed on the other half wave of the voltage to be limited.

The invention enables a reliable, sharp and distortionless amplitude limitation to be ob-' tained and, compared with other diode limiting systems, guarantees a reduction of interference Further the circuit is very simple.

In a preferred embodiment of the invention the diodes are so connected in tandem that either their cathodes or their anodes are interconnected by a condenser. In another favourable embodiment of the invention the diodes are connected in tandem by means of an intermediate amplifier in such a manner that the first diode is connected to the input and the second diode to the output of the amplifier. Further the diodes may be connected in tandem by means of a transformer.

The invention will be explained with reference to the drawings.

Fig. 1 shows a fundamental circuit embodying the invention.

Fig. 2 is for the purpose of explaining the operation of the circuit.

Fig. 3 shows a preferred embodiment of the invention.

Figs. 4 and 5 show modifications of this embodiment.

Fig. 6 shows another embodiment of the invention.

In all figures corresponding elements are provided with like reference characters.

Fig. 1 shows input terminals l and 2 connected to the first diode 5 by a condenser 3 and a resistance 4. The diode 5 is provided with an output resistance 6. The resistances 4 and 6 are interconnected by a condenser 1. The second diode I0 is connected in tandem with the diode 5 by means of a condenser 8, the arrangement being such that either the anodes or the cathodes of the two diodes are interconnected by the condenser 8. This means that the two diodes are connected in opposite directions.

. The second diode i is provided with an input resistance 9 and an output resistance l2 which are interconnected by a condenser H. In parallel to the resistance l2 are connected the condenser l3 and the resistance l4. The output terminals I5 and I5 are connected to this resistance. The diode 5 is supplied with a bias voltage from the terminals B1 and +13 of a direct current source, and the diode I0 is supplied with a bias voltage from the terminals -32 and +3 of the same source. The voltage between -Bi and +13 is higher than that between ---B: and +3.

The operation of the circuit may be explained with reference to Fig. 2. Diagram ashows a complete oscillation ii of the voltage to be limited. This voltage is supplied to the terminals l and 2 and may have interference peaks Z1 superimposed on the positive half wave and interference peaks Z: superimposed on the negative half wave. Diagram b shows the current-voltage characteristic of the diode 5 (id in terms of Ed). Diagram c shows the corresponding characteristic of the diode l0. As shown in diagram 1) the diode 5 is supplied with a biais voltage 65 of such magnitude that both halves of the alternating voltage f4 occurring across the resistance 4 are passed by this diode without rectification. The interference peak Z1, however, which is superimposed on the positive half wave is cut off. Consequently there will be transferred from the resistance 6 through the condenser 8 to the resistance 9 an oscillation one of the half waves of which is limited. As the diode I0 is connected'to the resistance 9 in a sense opposite to the connection of the diode 5 to the resistance 2 the oscillation just mentioned will be reversed with respect to. the diode l0,'as shown by It in diagram. The diode I0 is supplied with a bias voltage cm of such magnitude that the oscillation is is passed by this diode without rectification but that the interference peak Zz superimposed on the other negative half wave is cut oil. Consequently both half waves of the oscillation leaving the diode ID are limited. From the output terminals l5 and I6 the limited oscillation, which is represented by I15 in diagram d, is passed to the following stages of the circuit.

As indicated above the first diode 5 in the circuit of Fig. 1 is supplied with a higher bias voltage than the second diode III. The reason is that from an A. Cypoint of view the resistance 6 of the diode 5 is loaded by the circuits of the diode I 0. In case of the diode ill, however, the high impedances l3 and I! connected in parallel to the resistance l2 of the diode I 0 merely constitute a relatively low load for this diode. Consequently the alternating voltage supplied by the diode 5 will not appear in its full value across the resistance 9. Thus, the load on the diode 5 being higher than that on the diode III, the diode 5 is to be supplied with a higher bias than the diode Ill.

The bias voltage of the diodes may be supplied by any direct current source. It is recommendable to provide a source of current comprising a rectifier operated by the voltage to be limited. In that case the bias will be proportional to the amplitude of said voltage. Circuits embodying this feature are shown in Figures 3, 4 and 5. The limiting circuit employed in these figures is substantially the same as that in Fig. 1.

In the circuit of Fig. 3 the low frequency voltage to be limited is supplied from the input terminals I and 2 to the amplifier tube 20. This tube is preferably one having a wide range of control. The alternating voltage produced in the load resistance 2! of the tube 20 is supplied to the limiting circuit already described by way of the tap 22 and the condenser 3.

A double diode 25 is provided for producing the bias voltages for the diodes 5 and 0. The right hand section of the double diode 25 is connected to the point 23 of the resistance 2|, and the left hand section of the double diode 25 is connected to the point 24 of the resistance 2|, the point 24 having a lower potential than the point 23. The two connections just described comprise condensers 26 and 21. The rectified voltage produced in the right hand section of the diode 25 is taken from the load resistance 28 through a filter circuit comprising the resistance 29 and the condenser 30, and through a resistance 4 to the anode of the limiting diode 5. The rectified voltage produced in the left hand section of the diode 25 is taken from the load resistance 3! through a filter circuit comprising the resistance 52 and the condenser 33, and through a resistance l2 to the anode of the limiting diode l0. The circuit arrangement is such that the anodes of both limiting diodes 5 and III will receive a positive voltage with respect to their cathodes so that the diodes will be conductive. As the point 23 of the resistance 2| is of higher voltage than the point 24 of the same resistance the diode 5 will be supplied with a higher bias voltage than the diode ID.

If the input terminals l and 2 do not receive any low frequency voltage, that is if there is no signal voltage, the double diode 25 will not supply the diodes 5 and- Ill with bias voltages, so that there will be no current flowing in these tubes. If, however, the terminals 1 and 2 are supplied with signal voltage the limiting diodes 5 and III will receive a bias voltage and a signal voltage of corresponding value will be passed by these diodes in the manner described in connection with Fig. 2. The steady current of the limiting diodes, that is the diode current occurring even when there is no potential difference between the terminals l and 2, may be compensated for by supplying a compensating bias voltage to the diodes from a suitable point of any amplifier tube, for instance from the point 35 of the oathode resistance of the tube 20. In case the steady current may be neglected the rectifier means in the tube 20 may be combined. In that case the lower ends of the resistances l and I2 are earthed and the cathodes of the limiting diodes 5 and III are supplied with a smoothed negative bias voltage.

The bias voltage of the limiting diodes may also be supplied by a rectifier operated by the limited signal voltage. In that case the regulation is carried out backwards. If a fine regulation is not necessary the bias voltage for the limiting diodes may be derived from a constant voltage derived from the mains. In that case a variable resistance controlling the bias voltage may be adjusted together with the member controlling the signal strength. The circuit elements supplying the bias voltage may also be made independent of the receiving channel by providing an amplifier stage in parallel to the receiving channel and by rectifying the alternating voltage delivered by this amplifier.

Figures 4 and 5 show modifications of cult of Fig. 3. I

The circuit of Fig. 4 is different from that of Fig. 3 in that the load resistance inthe anode circuit of the tube 20 is replaced by a choke coil the cir- 40 across which the resistances 42, 43 and 44 are connected through a condenser 4|. The i required alternating voltages are taken from the points 22, 23 and 24. These points correspond to those bearing the same reference numbers in Fig. 3. 'It is an advantage of the circuit of Fig. 4 that there is no D. C. load in the anode circuit of the tube 20, so that the usful output voltage is not reduced.

The circuit of. Fig. is distinguished from that of Figs. 3 and 4 in that the anode circuit of the tube 20 is provided with a transformer having a primary winding 50 and a secondarywinding 5|. Th voltages to be passed to the double diode 25 producing the bias voltages for the diodes 5 and III are taken from the points 52 and 53 of the secondary transformer winding, and the alternating voltage directly supplied to the first diode 5 is taken from the point 54. The advantage of this circuit is that the transformer 50, '5! causes the anode alternating voltage of the tube 20 to be stepped up so that higher bias voltages may be obtained.

It is observed that Figures 4 and 5 have the double diode 25 reversed with respect to its position in Fig. 3, so that the left hand section of the double diode produces the bias voltage for the diode 5 and the right hand section for the diode In connection with Fig, 1 it has been stated that in View of the diiferent alternating current loads of the two limiting diodes the first diode 5 is to be supplied with a higher bias voltage than the second diode I. These different loads may be prevented by providing an amplifier tube between the two diodes. The diodes will t en be independent from each other so that the circuits of the second diode will not constitute an alternating current load for the first diode. The intermediate amplifier tube should preferably have a wide range of control so that even for great signal amplitudes there will be no distortion. Preferably the amplification of the intermediate tube is made substantially equal to the attenuation produced by the first diode so that this attenuation is compensated for. In that case both diodes may be supplied with the same bias voltage.

Fig. 6 shows an embodiment of a circuit having an intermediate tube. arranged between the diodes 5 and Ill. The input circuit of the amplifier comprises a coupling condenser Bi and a resistance 62, and the output circuit comprises a load resistance 53 and a condenser 64. In Figure 6 the sense of connection of the diodes 5 and I0 is the same in view of the fact that the signal voltage is reversed in the tube 60 so that the voltage across the resistance 9 is opposite to that across the resistance 4, which is necessary for a satisfactory operation of the limiting circuit (see Fig. 2)

Connected to the limiting diode I0 is an amplifier tube 65 for amplifying the limited alternating voltage. The tube 65 is provided with a cathode resistance 66,-and the anode circuit of An amplifier tube 60 is diodes. The bias voltage is thus supplied by a rectifier operated by the limited voltage, both limiting diodes receiving the same bias voltage.

In case of a limiting action depending on the degree of modulation it may be useful to have the bias voltage for the limiting diodes supplied by a rectifier connected to the anode circuit of a special amplifier tube controlled by the alterthe tube comprises the primary winding 61' of a nating voltage to be limited. In that case if there is no modulation the receiving channel remains completely closed, so that distortion will be prevented that might otherwise occur due to transients at the beginning of the modulation when using-a bias voltage supplied backwards. In case of a limiting action depending on the degree of modulation it may also be useful to cut oil the higher interference peaks, for instance those exceeding modulation, in any known manner in a stage preceding the limiting diodes.

The limiting circuit according to the invention may be employed in any receiver and in any stage of a receiver. Likewise it may be employed for limiting high frequency and video frequency currents. Instead of the two electrode diode tubes shown in the figures dry rectifiers or tubes having a grid but connected as diodes may be employed.

What I claim is:

1. In .a system for limiting the amplitudes of both half waves of an alternating current, two diodes connected in the circuit through which the current flows in such a manner that the current flows first through one diode and then the other and being connected to each other by means not transmitting direct current, means supplying to one diode a bias voltage causing the half waves of the alternating current of one polarity to be limited to a predetermined finite value and the half waves of the other polarity to be passed unlimited, and means supplying to the other diode a bias voltage causing the half waves of the first mentioned polarity to be passed unlimited and the half waves of the second mentioned polarity to be limited to a predetermined finite value, the bias voltages of the diodes being supplied by rectifying circuits operated by the alternating current to be limited.

2. In a system for limiting the amplitudes of both half waves of an alternating current, two

diodes connected in the circuit through which the current flows in such a mannergthat-the current flows first through one diode and then the other and being connected to each other by means not transmitting direct current, means supplying to one diode a bias voltage causing the half waves of the alternating current of one polarity to be limited toa predetermined finite value and the half waves of the other polarity to be passed unlimited, and means supplying to the other diode a bias.voltage causing the half waves of the first mentioned polarity to be passed unlimited and the half waves of the second mentioned polarity to be limited to a predetermined finite value, the bias voltages of the diodes being supplied by rectifying circuits operated by the limited alternating current.

3. In a system for limiting the amplitudes of both half waves of an alternating current, two diodes connected in the circuit through which the current flows in such a mannerthat the current flows first through one diode and then the other and being separated by an amplifier which is connected to the anode of one diode and the cathode. of the other, means supplying to one diode having a bias voltage causing the half waves of the alternating current of one polarity to be limited to a predetermined finite value and the half waves of the other polarity to be passed unlimited, and means supplying to the other diode a bias voltage causing the half waves of the first mentioned polarity to be passed unlimited and the half waves of the second mentioned polarity to be limited to a predetermined finite value.

4. In a system for limiting the amplitudes of both half waves of an alternating current, two diodes connected in the circuit through which the current flows in such a manner that the current fiows first through one diode and then the other and being connected together by means not transmitting direct current, means supplying to one diode a bias voltage causing the half waves of the alternating current of one polarity to be limited to a predetermined finite value and the half waves of the other polarity to be passed unlimited, and means supplying to the other diode a bias voltage causing the half waves of the first mentioned polarity to be passed unlimited and the half waves of the second mentioned polarity to be limited to a predetermined finite value, the

bias voltages of the diodes being supplied by,

rectifying circuits operated from an amplifier stage preceding the diodes.

5. In a, system for limiting the amplitudes of both half waves of an alternating current, two diodes connected in the circuit through which the current flows in such a manner that the current flows first through one diode and then the other and being connected together by means nottransmitting direct current, means supplying to one diode a bias voltage causing the half waves of the alternating current of one polarity to be limited to a predetermined finite value and the half waves of the other polarity to be passed unlimited, and means supplying to the other diode a bias voltage causing the half waves of the first mentioned polarity to be passed unlimited and the half waves of the second mentioned polarity to be limited to a predetermined finite value, the bias voltages of the diodes being supplied by rectifying circuits operated from an amplifier stage following the diodes.

6. In a system for limiting the amplitudes of both half waves of an alternating current, two diodes connected in the circuit through which the current flows in such a manner that the current flows first through one diode and then the other and being connected together by means not transmitting direct current, means supplying to one diode a bias voltage causing the half waves of the alternating current of one polarity to be limited to a predetermined finite value and the half waves of the other polarity to be passed unlimited, and means supplying to the other diode a bias voltage causing the half waves of the first mentioned polarity to be passed unlimited and the half waves of the second mentioned polarity to be limited to a predetermined finite value, the

bias voltages of the diodes being supplied by rectifying circuits operated from an amplifier stage preceding the diodes, certain of said rectifying circuits being supplied from the amplifier stage with a higher voltage than other rectifying circuits.

7. In a system for limiting the amplitude of both half waves of an alternating current, two diodes connected in the circuit through which the current fiows in such a manner that the current flows first through one diode and then the other and being connected together by means not transmitting direct current, means supplying to one diode a bias voltage causing the half waves of the alternating current of one polarity to be limited to a predetermined finite value and the half waves of the other polarity to be passed unlimited, and means supplying to th other diode a bias voltage causing the half waves of the first mentioned polarity to be passed unlimited and the half waves of the second mentioned po-' larity to be limited to a predetermined finite value, the bias voltages of the diodes being sup-v plied by rectifying circuits operated from an amplifier stage following the diodes, certain of said rectifying circuits being supplied from the amplifier stage with a higher voltage than other rectifying circuits.

8. In a system for limiting the amplitudes of both half waves of an alternating current, two diodes connected in the circuit through which the current flows in such a manner that the current flows first through one diode and then the other and being connected together by means not transmitting direct current, means supplying to one diode a bias voltage causing th half waves of the alternating current of one polarity to be limited to a predetermined finite value and the half waves of the other polarity to be passed unlimited, and means supplying to the other diode a bias voltage causing the half waves of the first mentioned polarity to be passed unlimited and the half waves of the second mentioned polarity to be limited to a predetermined finite value, the bias voltages of the diodes being supplied by rectifying circuits operated from an amplifier stage preceding the diodes, certain of said rectifying circuit being connected to the output circuit of the amplifier stage at a point different from the connection thereto of other rectifying circuits.

9. In a system for limiting the amplitudes of both half waves of an alternating current, two diodes connected in the circuit through which the current flows in such a manner that the current flows first through one diode and then the other and being connected together by means not transmitting direct current, means supplying to one diode a bias voltage causing the half waves of the alternating current of one polarity to be limited to a predetermined finite value and th half waves of the other polarity to be passed unlimited, and means supplying to the other diode a bias voltage causing the half waves of the first mentioned polarity to be passed unlimited and the half waves of the second mentioned polarity to be limited to a predetermined finite value, the bias voltages of the diodes being supplied by rectifying circuits operated from an amplifier stage following the diodes, certain of said rectifying circuits being connected to the output circuit of the fiows first through one diode and then the other and being connected together by means not transmitting direct current, means supplying to one diode a bias voltage causing the half waves of the alternating current of one polarity to be limited to a predetermined finite value and the half Waves of the other polarity to be passed unlimited,

and means supplying to the other diod a, bias voltage causing the half waves of the first mentioned polarity to be passed unlimited and the half waves of the second mentioned polarity to be limited to a predetermined finite value, the bias voltages of the diodes being supplied by an amplifier stage preceding the diodes and operating a common source of bias Voltage comprising a rectifying circuit connected to the output of the amplifier stage. i

11. In a system for limiting the amplitudes of both half waves of an alternating current, two

diodes connected in the circuit through which the current flowsin such a manner that the current fiows first through one diode and then the other and being connected together by means not transmitting direct current, means supplying to one diode a bias voltage causing th half waves of the alternating current of one polarity to be limited to a predetermined finite value and the half waves of the other polarity to be passed unlimited, and means supplying to the other diod a bias voltage causing the half waves of the first mentioned polarity to be passed unlimited and the half both halt waves of an alternating current, twodiodes connected in the circuit through which the current flows in such a manner that the current flows first through one diode and then the other and being connected together by means not transmitting direct current, means supplying to one diode a bias voltage causing the half waves of the alternating current 01 one polarity to be limited to a predetermined finite value and the half waves of the other polarity to be passed unlimited and means supplying to the other diode a bias voltage causing the half waves of the first mentioned polarity to be passed unlimited and the half waves of the second mentioned polarity to be limited to a predetermined finite value, the bias voltages of the diodes being proportional to thedegree of modulation of the alternating current.

13. In a system for limiting the amplitudes of both half waves of an alternating current, two diodes connected in the circuit through which the current flows in such a manner that the current flows first through one diode and then the other and being connected by a capacitor interconnecting the diodes in series opposition, means supplying to one diode a bias voltage causing the half wave of the alternating current of one polarity to be limited to a predetermined finite value and the half waves of the other polarity to be passed unlimited, and means supplying to the other diode a bias voltage causing the half waves of the first mentioned polarity to be passed unlimited and the half waves of the second mentioned polarity to be limited to a predetermined finite value, each of said means for supplying the bias voltages in cluding a resistor connected to its diode at the respective side of said capacitor, whereby the capacitor maintains the appropriate bias voltage at each of the diodes and passes the alternating current from one diode to the next.

GYQRGY ISTVAN DALLOS. 

